Mueller's blunders were obvious before he testified
His biggest blunder was the failure to investigate the Russian collusion of the Democrats in the publication of a fraudulent document like the Steel Dossier with the help of Fusion GPS. It was a smoking gun of Russian collusion by the Hillary Clinton campaign designed to attack the Trump campaign and also to be used as a reason for a coup attempt. When asked about it Mueller seemed absolutely clueless as does much of the liberal media. It looks as though Mueller's operation was a giant coverup of the real Russian collusion in the 2016 campaign.
A performance full of stumbles shakes a sterling Washington image
In what was likely to have been his final act on the national stage, Mueller’s halting testimony was at times painful to watch.